It is a goal of the tobacco industry to produce cigarettes which have a lower tendency to start fires. Such cigarettes are already part of statutory requirements in various countries and regions, for example, USA, Canada, Europe or Australia. To determine whether a cigarette has a reduced tendency to start fires, a test described in ISO 12863 or ASTM E2187 is used.
In this test, a smouldering cigarette is placed on a defined substrate, for example 10 layers of Whatman No. 2 filter paper, and observations are made as to whether the cigarette extinguishes before the entire visible tobacco part has smouldered away. This is called “self-extinguishment” (SE). The fraction of cigarettes which extinguish by themselves on the substrate is given as a percentage of the total number of tested cigarettes. In many cases the statutory requirements require that at least 30 out of 40 tested cigarettes have to extinguish, thus SE≧75%.
One way in which a cigarette self-extinguishes in this test consists in printing bands in cross-direction on the cigarette paper so that they are located in the circumferential direction on a cigarette manufactured from this cigarette paper. These bands can consist of any kind of materials; as an example, cellulose, cellulose derivatives, starch, starch derivatives or alginates have attained commercial relevance.
These bands primarily function by obstructing the access of oxygen to the glowing cone of the cigarette and thereby lead to the self-extinguishing of the cigarette. Since the access of oxygen during smouldering is primarily determined by the concentration difference between the inside of the cigarette and the surroundings, i.e. by diffusion, it is important to select a sufficiently low diffusion capacity for these bands.
The measurement of the diffusion capacity of such bands can be carried out with an appropriate measuring apparatus from the company Sodim (CO2 Diffusivity Meter). The diffusion capacity thereby describes a transport of gas through the cigarette paper driven by a concentration difference. It therefore indicates the volume of gas flowing through the paper per unit time, per unit area and per concentration difference and hence has the unit cm3/(cm2 s)=cm/s.
The low diffusion capacity necessary for self-extinguishing of the cigarette of the bands applied to the cigarette paper not only obstructs the transport of oxygen to the glowing cone, but also the transport of gases, in particular carbon monoxide, from the inside of the cigarette to the outside into the surroundings. Therefore, a higher carbon monoxide content is observed in the smoke of a self-extinguishing cigarette equipped with bands compared to the same cigarette without bands. However, the potentially increased fire safety should not increase the toxicity of the smoke, for which reason there is great interest in at least not increasing the carbon monoxide content in the smoke. Additionally, the carbon monoxide content in the smoke is also limited by statutory requirements in some countries.
A further disadvantage of self-extinguishing cigarettes is that the material which is applied in bands to the cigarette paper is burnt with the tobacco part of the cigarette, is smoked by the smoker and can thereby change the gustatory sensation of the cigarette. Here again, there is great interest in minimizing such changes in taste.
Finally, even if the cigarette has self-extinguishing properties on a suitable substrate, the cigarette should not extinguish by itself during normal smoking, for example, in the ashtray. Repeated lighting of a cigarette severely affects the acceptance of such cigarettes by the smoker and furthermore can have a negative influence on the composition of the smoke with respect to its toxicity.
A test to determine self-extinguishing during free smouldering, that is without substrate, has not been standardized and is also not part of statutory requirements. In most cases the cigarette is conditioned in accordance with ISO 3402 then lit and placed in a horizontal position so that air has unrestricted access to the cigarette from all sides by free convection. A certain number of cigarettes, for example 40 pieces, are tested according to this procedure and the fraction of cigarettes for which the entire visible tobacco part of the cigarette smoulders away, that is, the cigarette does not extinguish by itself, is determined. This fraction is often denoted as Free-Burn (FB) and given as a percentage. Although it is not part of statutory requirements, there is great interest in the industry for as few cigarettes as possible to be extinguished in this test, i.e. for the value of FB to be as high as possible. Generally, a value of at least 30% for FB is acceptable; preferably, the value should be 70% or higher.
In the prior art it is known that a minimum width for the bands of 4 mm is required to obtain self-extinguishing properties at all. In practice, however, it turns out in most cases that for compliance with the statutory requirements, typically 6 mm wide bands on the cigarette, printed on their entire surface, are required. The necessary width of the bands essentially depends on the selected tobacco blend. The distance between the bands results usually from the length of the tobacco part of the cigarette, as it is often a legal requirement that at least two bands should be present on the tobacco part.
Overall, one would like to achieve the partially conflicting goals of a high value for SE and a high value for FB at unchanged taste and at an at least not increased carbon monoxide content in the smoke. Several development attempts in this direction have, however, only provided partial progress.
A known approach that has gained commercial acceptance consists in dividing a 6 mm wide band into two bands of 3 mm each and separating them 1 mm from each other. This improves the FB, but the area covered by the bands on the cigarette paper is the same as for the normal 6 mm wide bands, so that the amount of material applied in the bands is at least not lower, for which reason no advantages with respect to the taste or the carbon monoxide content of the smoke can be expected.
Alternatives, in which the band edges are not made straight but, for example, are in the shape of waves to achieve improvements in the FB value, have proved to be of little value. They apply considerable amounts of material to the cigarette paper which cannot directly contribute to self-extinguishing. Thus, more material or a larger printed area is needed in order to comply with the statutory requirements regarding self-extinguishing properties. This can have a negative influence on the taste and the carbon monoxide content in the smoke. Similar approaches, in which the band edges are straight but in which the amount of applied material decreases from the centre of the band towards the band edges were not successful for the same reason.
Thus, there is a need for better reconciliation of the partially conflicting aims described above.